Billy Goat and Mariano’s are pretty close, a 5 or 10 minute walk. Chipotle is even closer. For the others, I would recommend taking a short cab from the Pier to lunch and then another cab to the stadium.

Billy Goat is fun (the subterranean one below Michigan Avenue). They server diner burgers, not a culinary hotspot. I take out-of-towners there.

During the work week (closed weekends), one of my favorite cheap lunches is another below-ground venue, the Berghoff Cafe downstairs from the Berghoff restaurant. Same food, but served from a cafeteria line.
Monday – Friday 11AM to 2PM.

Miller’s Pub is another old-school place.
Again, not a gourmet place. It’s kind of a time machine to the 1950s.
Every day 11AM to 4AM

I know some people don’t want to visit a chain while traveling, but I like Chipotle and there is one very close to Navy Pier.
291 E. Ontario
Every day, 11AM to 10PM